From late 1958 onwards, there should be a date stamped on the cantle plate (centrally, between the saddlebag loops) of the form XNN, where X is a letter denoting the quarter, and NN is the year (e.g. A79 for 1st quarter of 1979). It can be a bit indistinct under the rust, so have a look under a good strong light.
Is the style of the "Brooks Competition" embossed in the leather indicative of 1970s?
Nick